Bowling balls are provided with a number of holes, generally two finger holes and a thumb hole, which are drilled into the balls in order to permit the balls to be gripped in one hand and released with some degree of control during the act of bowling.
The position, location and sizes of the holes or bores will vary depending upon the size of the bowler's hand and fingers. Most bowlers purchase their own ball and have it custom-drilled to the size of their own hand and fingers. The holes must be sufficiently large to receive the fingers snugly but not so tightly that smooth release is interferred with.
One disadvantage of drilled holes or bores is that they are perfectly round, whereas it has been found preferable by most bowlers, particularly professional bowlers, that the release surface which is engaged by the fingerprint surface of the fingers should be flat in order to provide maximum "feel" and control during release of the ball.
Another disadvantage of drilled holes or bores is that such holes or bores have a hard inner surface which is uncomfortable to the touch, particularly in the case of professional bowlers who frequently bowl one hundred or more games each week for practice. Such lack of comfort interferes with control.
In order to overcome these problems, one or more diene rubber inserts have been developed and marketed for attachment within enlarged holes or bores on bowling balls. Such inserts are hose-type rubber elements which have a cylindrical outer surface, a uniform, relatively-thick wall and an inner surface having a flat portion in the area to be engaged by the finger-print surface of the fingers. These inserts are more comfortable than unlined holes and provide better ball control due to the flat surface. However, they are unsatisfactory for several reasons.
Firstly, such inserts are so long and large in diameter, due to the thick walls, that they require large diameter holes or bores to be drilled in the ball. This results in the removal of such an excess amount of weight from the ball that the uniform balance of the ball is disturbed or destroyed.
Secondly, such inserts have a finger-receiving opening or hole which is of uniform diameter from top to bottom. The finger of the bowler is inserted within said opening or hole to any desired depth. If the opening or hole tightly engages that finger, as can happen when the fingers swell during the progress of several games, the inside rubber surface of the insert can make airtight engagement with a substantial portion of the length of the finger, causing excessive resistance to removal of the finger during bowling due to the vacuum caused by the inability of external air to flow in and displace the area occupied by the finger, and also due to the length of the finger which is engaged by the rubber insert.